Field of the Invention
The present invention relates to a system which supplies gas enriched with nitrogen more than air to an aircraft fuel tank.
Description of the Related Art
Since an aircraft fuel tank is filled with vaporized fuel during a flight, it is necessary to prevent explosion of the fuel tank when, for example, the fuel tank is struck by lightning or a short occurs in the wiring. Thus, there has been proposed an explosion-proof system which supplies nitrogen enriched air (referred to as NEA below) having a higher nitrogen concentration and a lower oxygen concentration than air to the fuel tank.
While the air has an oxygen concentration of about 21%, the oxygen concentration of the NEA is set to, for example, 12% or less. To produce the NEA, an air separation module (ASM) that uses a permselective membrane having different permeability coefficients for oxygen molecules and nitrogen molecules is employed. Bleed air from a flight engine is used as a supply source of air to be supplied to the air separation module.
In the explosion-proof system in which the bleed air is used as the supply source of the NEA, it is necessary to increase the amount of bleed air from the engine so as to increase the amount of NEA to be supplied to the fuel tank. In this case, the fuel consumption of the engine is deteriorated. When the supply of the bleed air is increased, the oxygen concentration tends to become higher (the nitrogen concentration tends to become lower) in relation to the separation performance of the air separation module. For example, when the supply is increased to more than that by which an oxygen concentration of 10% is obtained, the oxygen concentration may be increased to 12% even by using the same air separation module.
U.S. Pat. No. 6,547,188 proposes a process for supplying NEA to an aircraft fuel tank.
U.S. Pat. No. 6,547,188 employs the concentration of inert gas in the NEA, and the supply of the NEA to the fuel tank as a target of control. In U.S. Pat. No. 6,547,188, the two control targets are adjusted according to a flight phase from takeoff to landing of the aircraft. In U.S. Pat. No. 6,547,188, the flight phase is divided into a phase from takeoff until entering a descent phase for landing through an ascent phase and a cruising phase (referred to as first phase below), and the descent phase (referred to as second phase below).
In U.S. Pat. No. 6,547,188, a relatively small amount of NEA is supplied in the first phase, and a relatively large amount of NEA is supplied in the second phase. It is described in U.S. Pat. No. 6,547,188 that the supply of the NEA to the fuel tank is increased in the second phase so as to compensate for an increase in the atmospheric pressure since the altitude is lowered in the second phase.
In U.S. Pat. No. 6,547,188, the concentration of the inert gas (typically, nitrogen gas) is set to a relatively high level of, for example, 98% in the first phase, and to a relatively low level of, for example, 86 to 95% in the second phase. It is described therein that the nitrogen concentration can be reduced in the second phase since the fuel tank contains a large amount of nitrogen gas in the first phase.
In accordance with U.S. Pat. No. 6,547,188, it is possible to prevent explosion of the fuel tank while improving the fuel consumption. In aircraft, however, there is always a demand for a decrease in the running costs.
The present invention has been made in view of such problems, and an object thereof is to provide a system which supplies NEA produced by using bleed air as an NEA supply source to a fuel tank, and which can further improve the fuel consumption.